Factor V Leiden thrombophilia is characterized by a poor anticoagulant response to activated protein C (APC) and an increased risk of venous thromboembolism. The term “factor V Leiden” refers to the specific G-to-A substitution at nucleotide 1691 in the gene for factor V that predicts a single amino acid replacement (Arg506Gln) at one of three APC cleavage sites in the factor Va molecule. Factor V Leiden is inactivated at a rate approximately ten times slower than normal factor V and persists longer in the circulation, resulting in increased thrombin generation and a mild hypercoagulable state reflected by elevated levels of prothrombin fragment F1+2 and other activated coagulation markers. Individuals heterozygous for the factor V Leiden mutation have a slightly increased risk for venous thrombosis; homozygous individuals have a much greater thrombotic risk.
Factor V Leiden is the most common hereditary blood coagulation disorder in the United States. It is present 5% of the in the Caucasian population and 1.2% of the African American population.
Factor V Leiden increases the risk of venous thrombosis 3-8 fold for heterozygous (one damaged gene inherited) and substantially more, 30-140 fold, for homozygous (two damaged genes inherited) individuals.
Deep venous thrombosis (DVT) with the attendant risk of pulmonary embolism and post phlebitic syndrome is a frequent complication in older patients who have undergone surgery, suffered trauma or who have serious illness such as malignancy or sepsis. In any category patients who are 40 years of age or older are considered to be at greatest risk. Also the longer the period of immobilization the greater the risk of DVT. Other factors that have been reported to contribute to development of DVT are obesity, prior history of DVT and smoking. While none of these factors alone or in combination will identify individual patients who will develop DVT, the incidence of DVT during the postoperative or post-traumatic period does correlate with the condition.
DVT has three major risks for the patient, two acute and one delayed. The acute problems are leg swelling, pain and tenderness and the risk of pulmonary embolism. In pulmonary embolism part of the thrombus breaks away and is carried to the lung where it can block a pulmonary artery causing respiratory distress in proportion to the amount of blockage, i.e., to the size of the embolus. Large emboli that block both pulmonary arteries cause immediate death. The delayed problem is the post phlebitic syndrome in which there is lower extremity pain or cramps at rest, leg edema, skin changes and skin breakdown causing chronic ulcers of the lower extremity. Clinicians have long known that the post phlebitic syndrome develops in a large percentage of patients who have DVT, especially those having extensive thrombus formation. Objective studies have shown that 1-10 years following the occurrence of DVT as much as 80% of patients will have both symptoms and abnormal venous hemodynamics (Lindner et al, 1986; Markel et al, 1992). While the post phlebitic syndrome is less dramatic than a major pulmonary embolus, it is a serious condition for the patients, resulting in much discomfort and expense.
In some patient groups DVT and pulmonary embolism are major causes of morbidity and mortality. Thromboembolism is a major cause of morbidity and mortality in patients with spinal cord injury. The prevalence of DVT has been reported to range from 47% (Merli et al, 1988) to 78% (Green et al, 1982). Of these 1 to 2% will die of pulmonary embolism (Green, 1991). Thrombosis usually occurs 1 to 3 weeks after injury, with a peak between days 7 and 9. The incidence of thromboembolic complications in patients undergoing surgery for fractured hip is high, ranging from about 40-60% (Powers et al, 1989; Fordyce and Ling, 1992; Turpie, 1991; Levine et al, 1991; Hull et al, 1990). In patients undergoing knee arthroplasty the incidence of DVT ranges from about 50% to 85% (Stulberg et al, 1984; Leclerc et al, 1992; Wilson et al, 1992). In gynecologic malignancy the incidence of DVT was 35% (Clarke-Person et al, 1984). The incidence of DVT in patients undergoing elective general abdominal surgery was about 9% in those without malignancy and about 11% in those with malignancies (Bergqvist et al, Seminars in Thromb & Hemost 16 Suppl 19-24, 1990).
For about 50 years efforts to prevent development of DVT and to treat those that do develop have focused on the judicious use of anticoagulants, first through full doses of oral anticoagulants and more recently through low dose heparin prophylaxis (Gallus, 1990). The aim has been to achieve a helpful degree of anticoagulation (prolongation of the clotting process) without causing hemorrhage. Low dose heparin has become the standard of comparison for other preventive methods since it is relatively safe and simple and prevents approximately 65% of subclinical thrombi found by leg scanning after elective general surgery. Postoperative death from pulmonary emboli may be reduced by 65% also.
However, there are clinical situations in which low dose heparin is less effective, most notable after orthopedic surgery where the use of more complex regimens, including adjusted dose heparin and various schedules of warfarin prophylaxis are appropriate. Several studies have shown that higher levels of anticoagulation are more effective than lower ones. However, if anticoagulation is too high, bleeding complications result.
Recently, double-stranded RNA molecules (dsRNA) have been shown to block gene expression in a highly conserved regulatory mechanism known as RNA interference (RNAi). WO 99/32619 (Fire et al.) discloses the use of a dsRNA of at least 25 nucleotides in length to inhibit the expression of the Factor V Leiden mutant gene in C. elegans. dsRNA has also been shown to degrade target RNA in other organisms, including plants (see, e.g., WO 99/53050, Waterhouse et al.; and WO 99/61631, Heifetz et al.), Drosophila (see, e.g., Yang, D., et al., Curr. Biol. (2000) 10:1191-1200), and mammals (see WO 00/44895, Limmer; and DE 101 00 586.5, Kreutzer et al.). This natural mechanism has now become the focus for the development of a new class of pharmaceutical agents for treating disorders that are caused by the aberrant or unwanted regulation of a gene.
Despite significant advances in the field of RNAi and advances in the treatment of thrombophilia, there remains a need for an agent that can selectively and efficiently silence the Factor V Leiden mutant gene using the cell's own RNAi machinery that has both high biological activity and in vivo stability, and that can effectively inhibit expression of a target Factor V Leiden mutant gene for use in treating thrombophilia.